Q1: What special factors should be considered in the strength design of thick-walled steel pipes?
The strength design of thick-walled steel pipes is much more complicated than that of thin-walled pipes, and the following factors should be considered: First, the three-dimensional distribution of hoop stress, radial stress and axial stress must be calculated, using the Lame formula instead of the simplified formula for thin walls; second, the performance difference in the thickness direction needs to be evaluated, especially the Z-direction anti-laminated tearing ability (section reduction rate ≥ 35%); third, creep rupture strength parameters (such as 100,000 hours of endurance strength) need to be introduced under high temperature conditions; fourth, fatigue analysis (S-N curve correction) is required for alternating loads; finally, for corrosive environments, the design wall thickness needs to add 2-5mm corrosion allowance to the calculated value. Standards such as ASME B31.3 and GB/T 20801 provide special design methods.
Q2: How to test the low-temperature impact toughness of thick-walled steel pipes?
The low-temperature impact test of thick-walled steel pipes has special requirements: the specimens need to be taken from 1/4 of the wall thickness (the most dangerous area), and samples are taken in the axial, circumferential and thickness directions; the standard Charpy V-notch specimens (10×10×55mm) need to be processed into full size, and when the wall thickness is insufficient, 7.5mm or 5mm small-size specimens can be used; the test temperature is set according to the minimum operating temperature (such as LNG pipes need to be tested at -196℃); the evaluation indicators include absorbed energy (such as X70 steel pipes -40℃≥60J) and shear area (≥85%); for extra-thick-walled pipes (>100mm), a drop weight tear test (DWTT) is also required to evaluate the crack arrest performance.
Q3: What are the characteristics of the residual stress distribution of thick-walled steel pipes?
The residual stress distribution of thick-walled steel pipes shows a significant gradient feature: the outer surface is usually compressive stress (-200~-400MPa), the inner surface is tensile stress (100~300MPa), and the stress value in the core is between the two; the residual stress in the weld zone of the welded thick-walled pipe can reach 80% of the yield strength; the stress distribution is affected by the manufacturing process, and the peak stress of the hot-rolled pipe is 30-40% lower than that of the cold-drawn pipe; residual stress can cause dimensional instability (such as deformation after machining) and stress corrosion cracking; the full-thickness stress distribution can be measured by synchrotron X-ray diffraction or deep hole method, providing a basis for subsequent heat treatment.
Q4: What is the failure mechanism of thick-walled steel pipes under high pressure?
The high-pressure failure of thick-walled steel pipes presents multi-mode characteristics: under the action of internal pressure, the inner wall usually begins to yield and gradually expands outward (Lame elastic solution); the explosion failure mode is divided into ductile fracture (mushroom-shaped port) and brittle fracture (straight crack); welded pipes mostly start cracking in the heat-affected zone, and there is an obvious plastic instability stage; fractures caused by creep cavity aggregation may also occur under high temperature and high pressure environments; for defective pipelines, fracture mechanics evaluation is required (such as CTOD value ≥ 0.15mm); modern design is generally based on the failure assessment diagram (FAD) method, which comprehensively considers the risks of plastic instability and brittle fracture.
Q5: How to evaluate the fatigue life of thick-walled steel pipes?
Fatigue assessment of thick-walled steel pipes requires special methods: first, measure the actual stress concentration factor (such as Kt ≥ 2.5 at the pipe joint) through strain gauges; then conduct full-size fatigue tests (usually 10^7 cycles) to establish a modified S-N curve; consider the wall thickness effect (for every 10mm increase in thickness, the fatigue strength decreases by 5-8%); the fatigue strength reduction factor (such as FAT80 grade) needs to be introduced into the welded joint; for corrosive environments, corrosion fatigue tests (frequency ≤ 1Hz) are required; based on Miner's linear cumulative damage theory, combined with the rain flow counting method, life prediction is performed, and the safety factor is usually 3-5.
